Abstract: A monitor probe for a communications signalling network, such as an SS7 network, is arranged to determine its own location on the signalling network by watching for a particular type of message which carries the required information in its routing label. The routing label of a message is the part of the message indicating the source and destination points for the message as well as providing link-related information for cases where more than one link connects two signalling points. Whilst the routing label of most messages on an SS7 network does not give a reliable indication of the absolute identity of the link on which the message was monitored, in the case of signalling link test messages, the routing label contains the identity of the two points at the end of the link on which the message was monitored as well as the identity of the link as between that link and any other parallel link; these items of information form an absolute identifier for the link which the message was monitored.